Hi Karst71,
the second method has virtually no effect on flex. Given a full wood core and epoxy & fibre glass on top, the main important factor regarding flex is the thickness of the board. If you build an uneven shape like the redecessed deck, you have to take into account the thickness of the rim and the thickness of the center. the grain direction is rather important when it comes to breaking strength. Reason: the main load goes to the surface (fibre glass), not the core. How much glass you have to put on, depends on board thickness (where it flexes) and core density (where you stand on it).
Sure it depends on the wood as well, paulownia has different flex from balsa or bamboo, but this, in the end, comes down to density: You will not want to build a full body bamboo board, too heavy. Balsa is 100-170kg/m3, paulownia 200-250kg/m3, bamboo 350-400kg/m3.
So in a big picture, shape and thickness are important. Density is important if you want to go thicker and / or lighter. If you're able to distinguish equal boards, one made of paulownia and one of hard balsa, you are such a good and experienced rider, you could as well go out on a McDonalds tray and make it shine.
A fad: